1. Field of the Invention
The present invention relates to a liquid jet head for forming an image, a character, or a thin film material on a recording medium by discharging liquid from a nozzle, and relates to a method of manufacturing the liquid jet head and to a liquid jet apparatus using the liquid jet head.
2. Description of the Related Art
In recent years, there has been used an ink jet type liquid jet head for discharging ink droplets on recording paper or the like to render a character or graphics or for discharging a liquid material on a surface of an element substrate to form a pattern of a functional thin film. In such a liquid jet head, ink or a liquid material is supplied from a liquid tank via a supply tube to the liquid jet head, the ink is caused to fill minute space formed in the liquid jet head, and a capacity of the minute space is momentarily changed according to a drive signal to discharge liquid droplets from a nozzle which communicates to a groove.
FIG. 11 is an exploded perspective view of an ink jet head 51 of this type (FIG. 1 of JP 2000-108361 A). The ink jet head 51 includes: an actuator substrate 52 including a plurality of grooves 53 formed in a surface thereof; a cover plate 56 bonded onto the actuator substrate 52 so as to cover the plurality of grooves 53; a manifold 57 bonded to a rear end of the actuator substrate 52, for supplying ink into the plurality of grooves 53; and a nozzle plate 58 bonded to a front end of the actuator substrate 52, the nozzle plate 58 including nozzles 58a for discharging the ink.
The actuator substrate 52 and partition walls 54 are made of a piezoelectric material, and the partition walls 54 are subjected to polarization processing in a normal direction of the substrate surface. Electrodes 55 are respectively formed on both side surfaces of each partition wall 54 so as to sandwich the partition wall 54. By giving a drive signal to the electrodes 55, the partition wall 54 slips to be deformed in a thickness direction, to thereby change internal volumes of the grooves 53. Thus, the ink filled in the grooves 53 is caused to jet from the nozzles 58a, and is recorded on the recording medium.
A bending point when the partition wall 54 slips to be deformed in the thickness direction is situated at substantially half a height from a bottom surface of each groove 53 to a top surface of each partition wall 54. With this configuration, it is possible to most efficiently deform and drive the partition wall 54. For that reason, the electrode 55 to be formed on each surface of the partition wall 54 is formed from the bottom surface of the groove 53 to ½ of the height of the partition wall 54, or formed from ½ of the height of the partition wall 54 to the height of the top surface of the partition wall 54. When widths in a depth direction of the electrodes 55 vary in every groove 53, ink discharge performance varies in every nozzle 58a. The recording medium, on which ink droplets are jetted, moves. Accordingly, when flying rate of the ink droplets varies, the jetted positions vary, which leads to degradation in printing quality. Therefore, the electrodes 55 to be formed on the side surfaces of the partition walls 54 need to be formed into the same shape in the respective grooves 53.
In JP 2000-108361 A, metal electrodes are formed on the entire surface including the side surfaces of the partition wall 54 and the bottom surface of the groove 53 by electroless plating processing. Then, a laser beam is irradiated from a direction that is oblique in the direction orthogonal to the grooves 53 with respect to the normal of the surface of the actuator substrate 52, and the upper half of the metal electrode formed on one side surface of the partition wall 54 is removed. Next, the upper half of the metal electrode formed on the other side surface is removed by irradiating the laser beam from the opposite oblique direction. If the metal electrodes are to be removed together by irradiating the laser beam to a large area at the time of removal, an incident angle of the laser beam irradiated to the surface differs in every position, and hence the electrodes vary in width. In order to avoid this, it is necessary to irradiate the laser beam to a small area by limiting the irradiation range.
JP 05-318741 A describes another method of forming the electrodes 55. After a plurality of grooves are formed in an actuator substrate made of a piezoelectric material, a target wire is inserted into each of the grooves, the target wire having a diameter substantially equal to the width of each groove. By irradiating an inert gas ion beam from a direction of upper openings of the plurality of grooves, the target wire embedded in each of the grooves is sputtered. In this way, metal particles sputtered from the target wire adhere to upper portions of side wall surfaces. After that, the target wire is taken out of each of the grooves.
As another electrode forming method, there is known an oblique deposition of depositing a conductive material obliquely. FIG. 12 illustrates a method of forming drive electrodes by the oblique deposition on the side surfaces of the partition walls 54 made of the piezoelectric material. The actuator substrate 52 is inserted into a chamber of a vacuum deposition device. First, assuming that an inclination angle θ is formed between an evaporation source 59 and a normal direction n of the surface on which the partition walls 54 are formed, the actuator substrate 52 is placed so that the evaporation source 59 is substantially orthogonal to a longitudinal direction of the partition walls 54 (position P1). Then, metal, for example, aluminum is deposited from the evaporation source 59 onto one side surface of each of the partition walls 54. Next, the actuator substrate 52 is placed so that the evaporation source 59 forms an inclination angle −θ with respect to the normal direction n (position P2). Then, the metal is deposited from the evaporation source 59 onto the other side surface of each of the partition walls 54. In this way, it is possible to form each electrode 55 on the top surface side situated above substantially half the height of the partition wall 54.
The electrodes 55 which are formed on the side surfaces of the partition walls 54 need to be formed into the same shape in the respective partition walls 54. In JP 2000-108361 A, in order to form the electrodes 55 into the same shape, the laser beam needs to be irradiated to every side surface of each of the partition walls 54, which requires a greater amount of time for patterning the electrodes 55 as the number of the grooves 53 of the ink jet head increases. Consequently, mass production performance is reduced. Further, a metal material is scattered to the surroundings through irradiation of the laser beam, and the scattered metal material adheres to the grooves 53 again to cause a short circuit and clogging of the nozzles. Further, in the method described in JP 05-318741 A, it is necessary to embed a large number of target wires into a large number of the narrow grooves 53, respectively, the target wires each having a diameter substantially equal to the width of the groove 53. Thus, the mass production performance is low, and the method is not realistic.
FIGS. 13A to 13C are sectional schematic views of the actuator substrate 52 in which the electrodes 55 are formed on the side surfaces of the partition walls 54 by the method illustrated in FIG. 12. FIG. 13A is an overall view of the actuator substrate 52, and FIGS. 13B and 13C are partial sectional views respectively illustrating the left and right sides of the actuator substrate 52. At a left end portion of the actuator substrate 52, the electrodes 55 formed on the side surfaces of each partition wall 54 are formed deeper on the left side surface than on the right side surface. Further, at a right end portion of the actuator substrate 52, the electrodes 55 formed on the side surfaces of each partition wall 54 are formed deeper on the right side surface than on the left side surface. This is because the direction of the evaporation source 59, that is, the inclination angle θ changes depending on the position of the surface of the actuator substrate 52. In other words, at a position near the evaporation source 59, the inclination angle θ is small and the electrode 55 is formed on a deeper portion of the side surface, whereas at a position distant from the evaporation source 59, the inclination angle θ is large and the electrode 55 is formed on a shallower portion of the side surface.
As described above, the electrodes 55 formed on the partition walls 54 differ in depth depending on the position of the surface of the actuator substrate 52. FIG. 14 shows a relation between a nozzle No (nozzle position) and liquid droplet discharge rate (relative value) of the ink jet head when the electrodes 55 are formed by the oblique deposition illustrated in FIGS. 13A to 13C. As shown in FIG. 14, the nozzles situated at the center portion have higher liquid droplet discharge rate than the nozzles situated at the peripheral portion. This is because an electric field is applied more efficiently to the partition walls 54 situated at the center portion than those situated at the peripheral portion. However, such variations in liquid droplet discharge rate cause degradation in printing quality.
Specific description is made with reference to FIGS. 15A to 15D and FIGS. 16A and 16B. FIGS. 15A to 15D are sectional schematic views of a discharge channel formed by the groove 53 formed in the actuator substrate 52 and by the cover plate 56 bonded onto the top surface of the actuator substrate. In FIGS. 15A to 15D and FIGS. 16A and 16B, the partition walls 54 are made of the piezoelectric material, and are uniformly subjected to polarization processing in a perpendicular direction (height direction of the partition walls 54). FIGS. 15A and 15B illustrate a case where the electrode 55 is formed on each side surface on an upper side situated above substantially half a height h of the partition wall 54, and FIGS. 15C and 15D illustrate a case where the electrode 55 is formed to extend over a lower side situated below substantially half the height h of the partition wall 54.
As illustrated in FIG. 15A, when a voltage is applied to terminals Ta and Tb, the electric field is applied in the thickness direction of the partition walls 54. Then, slip stress (shear stress) S is generated on the surface of each partition wall 54 on the electrode 55 side to bend the center portion of the partition wall 54 to an outer side Ou. In addition, when a polarity of the applied voltage is reversed, as illustrated in FIG. 15B, the direction of the slip stress S is reversed so that the center portion of the partition wall 54 is bent to an inner side In. In this way, by deforming and driving the partition walls 54, the ink filling a discharge channel C is discharged from the nozzle 58a. 
Next, description is made of the case where the electrodes 55 extend across the center portion of the partition wall 54 over the lower side thereof. In FIG. 15C, by applying the voltage to the terminals Ta and Tb, the electric field is applied to the partition walls 54. Then, similarly to the case of FIG. 15A, the slip stress S is generated at the upper half of each partition wall 54, to thereby bend the partition wall 54 to the outer side Ou. Meanwhile, the slip stress generated in an electric field application region on the lower side of the partition wall 54 attempts to bend the partition wall 54 to the inner side In. Therefore, a force of bending the partition wall 54 to the outer side Ou is reduced, with the result that a deformation amount of the partition wall 54 is reduced and power consumption is increased. When the polarity of the applied voltage is reversed, as illustrated in FIG. 15D, the direction of the slip stress S is reversed to attempt to bend the center portion of the partition wall 54 to the inner side In at the upper half of the partition wall 54. However, the slip stress generated on the lower side of the partition wall 54 attempts to bend the partition wall 54 to the outer side Ou, and hence similarly to the case of FIG. 15C, the deformation amount of the partition wall 54 is reduced and the power consumption is increased.
FIGS. 16A and 16B are sectional schematic views of the actuator substrate 52, and illustrate shapes of the electrodes 55 with respect to positions of the partition walls 54. FIG. 16A illustrates a case where all the electrodes 55 are formed on the upper side situated above the upper halves (½) h of the partition walls 54, and FIG. 16B illustrates a case where all the electrodes 55 are formed to extend over the lower halves of the partition walls 54. As illustrated in FIG. 16A, among the electrodes 55 to be formed on the left side surfaces of the respective partition walls 54, the electrode 55 of the partition wall 54 situated at the left end portion is deepest, and the electrodes 55 become gradually shallower toward the right end portion. Meanwhile, among the electrodes 55 to be formed on the right side surfaces of the respective partition walls 54, the electrode 55 of the partition wall 54 situated at the left end portion is shallowest, and the electrodes 55 become gradually deeper toward the right end portion. As a result, an area of an upper half of each electric field application region Sa, in which the right and left electrodes 55 overlap each partition wall 54, becomes widest at the center portion of the actuator substrate 52 and becomes narrower toward both end portions of the actuator substrate 52. It can be understood that, because the areas of the electric field application regions Sa change depending on the positions of the partition walls 54, the discharge rate becomes highest at the center portion as shown in FIG. 14, and becomes lower toward both the end portions.
In the case where the electrodes 55 extend over the lower halves of the partition walls 54, as illustrated in FIG. 16B, the areas of the electric field application regions Sa at the upper halves of the partition walls 54 are constant, whereas electric field application regions Sb at the lower halves thereof become widest at the center portion, and become narrower toward the peripheral portion. In other words, the slip stress at the upper halves of the partition walls 54 is equal among the respective partition walls 54, whereas the slip stress at the lower halves thereof, which functions as a brake with respect to deformation of the partition walls 54 resulting from the above-mentioned stress, is highest at the center portion of the actuator substrate 52, and gradually decreases toward the peripheral portion. Thus, even in the case of FIG. 16B, discharge rate of the ink droplets discharged from the nozzles 58a is not constant. In addition, deformation drive of the partition walls 54 is accelerated and decelerated at the same time, and hence energy is consumed wastefully. In the actual actuator substrate 52 illustrated in FIGS. 13A and 13B, in order to prevent the electrodes 55 from ranging in the depth direction at the time of oblique deposition, and prevent the electrodes 55 from being deposited on the lower side than the height h/2 and functioning as the brake at the time of deformation drive of the partition walls 54, all the electrodes 55 to be formed on the side surfaces of the partition walls 54 are formed on the upper side than the height h/2.
As described above, in the actuator substrate 52 that is uniformly polarized in an upright direction of the partition walls 54, if the areas of the electric field application regions are not constant in the respective partition walls 54, it is impossible to ensure equality of the discharge rate. Further, in order to increase electrostrictive efficiency, and to lower the applied voltage so as to reduce load applied to the drive circuit side, it is necessary that each electrode 55 does not extend over a portion situated below the height h/2 of the partition wall 54 in the depth direction, and that the upper half of the electric field application region Sa is formed as wide as possible. Thus, it has been extremely difficult to form the electrode.